Manufacture of silica waveguides with minimal absorption

ABSTRACT

An improved high temperature chemical treatment of deposited silica films wherein they are subjected to a reactive ambient comprising hydrogen and oxygen atoms. This method results in better elimination of residual undesirable oscillators so as to provide improved optical quality silica waveguides with reduced optical absorption.

FIELD OF THE INVENTION

[0001] This invention relates to an improved method for manufacturingsilica waveguides with minimal absorption.

BACKGROUND OF THE INVENTION

[0002] The manufacture of optical devices which employ silicawaveguides, such as optical Multiplexers (Mux) and Demultiplexers(Dmux), entails depositing silica films onto a silicon wafer. The silicafilms are ideally of optical quality, characterised in that they aretransparent in the 1.30 μm bi-directional narrow optical band and/or inthe 1.55 μm video signal optical band¹. Such optical quality silicafilms are extremely difficult to produce in reality because hydrogen andnitrogen atoms are typically present in the films. These impurities inthe silica films result in excessive optical absorption in the 1.30 and1.55 μm optical bands.

[0003] Fourier Transform Infrared (FTIR) spectroscopy can be used tomonitor the quality of optical silica films. The FTIR spectra of opticalquality silica films, containing no undesirable optical absorptionpeaks, are characterised by the presence of only four fundamentaloptical absorption peaks: (1) an intense and small Full Width at HalfMaximum (FWHM) Si—O—Si “rocking mode” absorption peak ranging between410 and 510 cm⁻¹, centred at 460 cm⁻¹ (21.739 μm); (2) a small FWHMSi—O—Si “bending mode” absorption peak ranging between 740 and 880 cm⁻¹,centred at 810 cm (12.346 μm); (3) an intense and small Full Width atHalf Maximum (FWHM) Si—O—Si “in-phase-stretching mode” absorption peakranging between 1000 and 1160 cm⁻¹, centred at 1080 cm⁻¹ (9.256 μm)indicating stoichiometric silica films with the optimum Si—O—Si bondangle of 144° and the optimum density; and (4) an almost eliminatedSi—O—Si “out-of-phase-stretching mode” absorption peak ranging between1080 and 1280 cm⁻¹, centred at 1180 cm⁻¹ (8.475 μm), as compared to theSi—O—Si in-phase-stretching mode absorption peak.

[0004] The position in the infrared spectrum of these four fundamental(first mode) infrared absorption peaks, respectively centered at 21.739μm, 12.346 μm, 9.256 μm, and 8.475 μm, is far away from the infraredbands of interest at 1.30 and 1.55 μm. However, residual absorption ofoptical quality silica is never completely eliminated because the higherharmonics of these four residual optical absorption peaks do cause smallresidual optical absorption peaks in the 1.30 and 1.55 μm optical band.The very high harmonics (i.e. very little absorption effect) fallingwithin this range are: the sixth (1.302 to 1.543 μm) and seventh (1.116to 1.323 μm) harmonics of the Si—O—Si “out-of-phase-stretching mode”infrared absorption peak; the sixth (1.437 to 1.667 μm) and seventh(1.232 to 1.429 μm) harmonics of the Si—O—Si “in-phase-stretching mode”infrared absorption peak; the eighth (1.420 to 1.689 μm) and ninth(1.263 to 1.502 μm) harmonics of the Si—O—Si “bending mode” infraredabsorption peak; and the thirteenth (1.508 to 1.876 μm), fourteenth(1.401 to 1.742 μm) and fifteenth (1.307 to 1.626 μm) harmonics of theSi—O—Si “rocking mode” infrared absorption peak.

[0005] The FTIR spectra of optical quality silica films are alsocharacterised by a net separation between the Si—O—Si“in-phase-stretching mode” absorption peak (1080 cm⁻¹) and the Si—O—Si“bending mode” absorption peak (810 cm⁻¹) with a deep valley between 850and 1000 cm⁻¹.

[0006] Silica films may be deposited onto a silicon wafer using a silane(SiH₄) and nitrous oxide (N₂O) gas mixture at a low temperatureaccording to the following reaction:

SiH₄(g)+2N₂O(g)→SiO₂+2N₂(g)+2H₂(g)

[0007] Theoretically, it is possible to achieve optical quality silicafilms from this reaction. However, in reality, numerous side reactionsoccur, forming a mixture of undesirable Si—O_(x)—H_(y)—N_(z) compounds.For example, FIG. 1 presents the various potential Si—O_(x)—H_(y)—N_(z)compounds that may result from the combination of silane (SiH₄) andnitrous oxide (N₂O) gases. It shows 35 products that could be found insilica films deposited from a silane (SiH₄) and nitrous oxide (N₂O) gasmixture. N₂, O₂, HNO, NH₃, H₂O, and H₂ gaseous by-products areeliminated from the surface or from the micro-pores of the silica filmsduring these chemical reactions. As a result of the production of theseside-products, the incorporation of oxygen atoms, a key factor toachieve optical quality silica, competes with the incorporation ofnitrogen and hydrogen atoms in the silica films. Thus, the silica filmsas deposited on the silicon wafer are not optical quality silica films,due to the absorption by the undesirable Si—O_(x)——H_(y)—N_(z) compoundsformed.

[0008] To resolve this problem caused by Si—_(O)x—H_(y)—N_(z) impuritiesin the films, techniques have been used wherein the silica films aresubject to a high temperature (typically, between 600 and 1350° C.)thermal treatment under vacuum, argon (Ar), or a nitrogen atmosphere asa means for reducing the optical absorption of silica films in the 1.30and 1.55 μm optical regions². In general, the higher the temperature ofthis high temperature thermal treatment, the lower the opticalabsorption of the silica films. However, unlike fused silica opticalfibres, that are heated at a temperature exceeding about 2000° C. duringthe drawing process, the high temperature thermal treatment of thesilica films on silicon wafers is performed at a temperature rangingfrom 600° C. to a maximum temperature of about 1350° C., close to thefusion point of the silicon wafer. The temperature is typically limitedby the high compressive mechanical stress induced in the silica filmsfrom the difference of thermal expansion between the silica films andthe underlying silicon wafer. This temperature limitation results insilica films with undesirable residual infrared oscillators and in theirassociated undesirable residual optical absorption peaks in the 1.30 and1.55 μm wavelength optical bands.

[0009] Thus, using a high temperature thermal treatment in the presenceof nitrogen on the thirty-five products of silane and nitrous oxidegiven in FIG. 1, results in a maximum of only twelve of the thirty-fivepotential Si—O_(x)—H_(y)—N_(z) products being converted to SiO₂. Thesame twelve compounds could also lead to the formation of SiO₂ in aninert (Ar) atmosphere or under vacuum, since in none of these twelvechemical reactions is nitrogen incorporated.

[0010] Following a high temperature thermal treatment in a nitrogenatmosphere, the other twenty-three Si—O_(x)—H_(y)—N_(z) potentialcompounds can lead to the formation of: SiNH, SiN₂, SiOH₂, SiONH, andSiON₂ Therefore, high temperature thermal treatments under nitrogen,argon, or in a vacuum are incapable of transforming twenty-threepotential initial Si—O_(x)—H_(y)—N_(z) products formed from the reactionof silane and nitrous oxide into SiO₂. Thus, the silica films thatresult from these high temperature thermal treatments under nitrogen,argon, or in a vacuum are composed not only of SiO₂, but are solidmixtures of six possible compounds: SiO₂, SINH, SiN₂, SiOH₂, SiONH andSiON₂. Gaseous byproducts that result from the thermal decomposition ofsilica films are: nitrogen (N₂), hydrogen (H₂), and ammonia (NH₃).

[0011]FIG. 3 lists some FTIR fundamental infrared absorption peaks andtheir corresponding higher harmonic peaks associated with SiO₂, SiNH,SiN₂, SiOH₂, SiONH, and SiON₂. The higher harmonics of the absorptionpeaks corresponding to these six residual potential compounds contributeto the optical absorption in the 1.30 and 1.55 μm optical bands, asfollows: the second vibration harmonics of the HO—H oscillators intrapped water vapour in the micro-pores of the silica films (3550 to3750 cm⁻¹) increases the optical absorption near 1.333 to 1.408 μm; thesecond vibration harmonics of the SiO—H oscillators in the silica films(3470 to 3550 cm⁻¹) increases the optical absorption near 1.408 to 1.441μm; the second vibration harmonics of the SiN—H oscillators in thesilica films (3380 to 3460 cm⁻¹) increases the optical absorption near1.445 to 1.479 μm; the second vibration harmonics of the SiN—Hoscillators in the silica films (3300 to 3460 cm⁻¹) increases theoptical absorption near 1.445 to 1.515 μm; the third vibration harmonicsof the Si—H oscillators in the silica films (2210 to 2310 cm⁻¹)increases the optical absorption near 1.443 to 1.505 μm; the fourthvibration harmonics of the Si═O oscillators in the silica films (1800 to1950 cm⁻¹) increases the optical absorption near 1.282 to 1.389 μm; andthe fifth vibration harmonics of the N═N oscillators in the silica films(1530 to 1580 cm⁻¹) increases the optical absorption near 1.266 to 1.307μm. The negative effects of these the oscillators on the opticalproperties of silica films are reported in the literature.³

[0012] Thus, this high temperature thermal treatment of silica filmsunder vacuum, argon, or nitrogen, does not provide a very efficient wayto eliminate the excessive absorption at various wavelengths in the 1.30and 1.55 μm optical bands.

SUMMARY OF THE INVENTION

[0013] An object of the invention is to alleviate the afore-mentionedproblems.

[0014] 2. In one aspect, the invention provides a method of making ahigh optical quality silica film, comprising: subjecting a substrate toa gaseous mixture of silane and nitrous oxide to deposit said film onsaid substrate in accordance with the reaction

SiH₄(g)+2N₂O(g)→SiO₂+2N₂(g)+2H₂(₉)

[0015] and subsequently subjecting said deposited film to a reactiveatmosphere containing hydrogen and oxygen atoms to chemically transformimpurities resulting from the reaction into pure silica.

[0016] In another aspect the invention provides a method for reducingthe optical absorbance of a silica film coated on a substrate,comprising: subjecting the silica film coated on the substrate to atemperature of about 6000 to about 1000° C. under nitrogen, an inertatmosphere, or under vacuum; increasing the temperature to a maximumtemperature of at most 1350° C.; exposing the silica film to a reactiveatmosphere comprising oxygen and hydrogen atoms by replacing thenitrogen, inert atmosphere or vacuum by the reactive atmosphere;removing the silica film from the reactive atmosphere by replacing thereactive atmosphere with nitrogen, an inert atmosphere, or vacuum;decreasing the temperature from the maximum temperature to about 1000°to about 600° C.; and recovering the silica film coated on thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, in which:

[0018]FIG. 1 is a table of thirty-five compounds that could be found insilica films as a result of chemical reactions in a silane (SiH₄) andnitrous oxide (N₂O) gas mixture;

[0019]FIG. 2 is a table of the possible chemical reactions that mayensue from exposure of the thirty-five of Figure compounds to nitrogenat very high temperature;

[0020]FIG. 3 is a table of the FTIR fundamental infrared absorptionpeaks and their corresponding higher harmonics peaks associated with thesix residual potential compounds that result from a high temperaturethermal treatment of silica films under nitrogen, argon, or undervacuum;

[0021]FIG. 4 is a table of the possible chemical reactions that mayresult from the exposure of the thirty-five potential compounds to a gasmixture of hydrogen (H₂), oxygen (O₂), water vapour (H₂O), at very hightemperature;

[0022]FIG. 5 is the FTIR spectra of PECVD silica films before and aftera three hours long high temperature thermal treatment in a nitrogenambient at a temperature of either 600°, 700°, 800°, 900°, 1000°, or1100° C.

[0023]FIG. 6 is an expanded view of the FTIR spectra of FIG. 5 in theregion between 810 to 1000 cm⁻¹.

[0024]FIG. 7 is an expanded view of the FTIR spectra of FIG. 5 in theregion between 1500 to 1600 cm⁻¹.

[0025]FIG. 8 is an expanded view of the FTIR spectra of FIG. 5 in theregion between 1700 to 2200 cm⁻¹.

[0026]FIG. 9 is an expanded view of the FTIR spectra of FIG. 5 in theregion between 2200 to 2400 cm⁻¹.

[0027]FIG. 10 is an expanded view of the FTIR spectra of FIG. 5 in theregion between 3200 to 3900 cm⁻¹.

[0028]FIG. 11 is the FTIR spectra of PECVD silica films before and aftera three hour long high temperature chemical treatment in a H₂, O₂, andH₂O atmosphere at a temperature of 8000, 9000, 10000, or 1100° C.Comparison is made to the FTIR spectrum of PECVD silica films after athree hour high temperature thermal treatment under nitrogen (describedin Comparative Example 1), at a temperature of 1100° C.

[0029]FIG. 12 is an expanded view of the FTIR spectra of FIG. 11 in theregion between 810 to 1000 cm⁻¹.

[0030]FIG. 13 is an expanded view of the FTIR spectra of FIG. 11 in theregion between 1500 to 1600 cm⁻¹.

[0031]FIG. 14 is an expanded view of the FTIR spectra of FIG. 11 in theregion between 1700 to 2200 cm⁻¹.

[0032]FIG. 15 is an expanded view of the FTIR spectra of FIG. 11 in theregion between 2200 to 2400 cm⁻¹.

[0033]FIG. 16 is an expanded view of the FTIR spectra of FIG. 11 in theregion between 3200 to 3900 cm⁻¹.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0034] The method of this invention comprises subjecting a silica film,coated on a substrate, to three stages: a first dry stage, a wet stage,and a second dry stage. During the two dry stages the silica filmdeposited on a substrate is subject to high temperatures under anitrogen or inert atmosphere, or under vacuum. During the wet stage the“dry” atmosphere is replaced by a “wet” atmosphere comprising a mixtureof hydrogen gas and oxygen gas which also produces, by thermaldecomposition at such a high temperature, some water vapour(2H₂+O₂→2H₂O). These reactive gases, H₂, O₂, and H₂O, are at the basisfor the elimination of the undesirable oscillators of the variousside-products of FIG. 1 and of the achievement of extremely high qualityoptical silica films.

[0035] As discussed above, silica films deposited onto a substrate froma silane and nitrous oxide gas mixture at low temperature contain amixture of silicon dioxide and up to 35 undesirable Si—O_(x)—H_(y)—N_(z)compounds listed in FIG. 1. FIG. 4 lists the possible chemical reactionsthat may result from the exposure of the thirty-five potential compoundsto a gas mixture composed of hydrogen, oxygen, and water vapor at veryhigh temperature. As shown in FIG. 4, each one of the thirty-fiveSi—O_(x)—H_(y)—N_(z) potential compounds can lead to the formation ofpure SiO₂ after a high temperature chemical treatment in a gas mixturecomposed of hydrogen, oxygen, and water vapor at very high temperature.Thus, the silica films that result from these high temperature chemicaltreatments are potentially pure SiO₂. FTIR spectra can be used tomonitor the transformation of these treated silica films into pure SiO₂.

[0036] The high temperature chemical treatment method of the inventionis preferably carried out in a vertical or horizontal diffusion furnace,more preferably in a BTI horizontal furnace.

[0037] The substrate is preferably a wafer of 100 mm, 125 mm, 150 mm,200 mm, or 300 mm, more preferably of 150 mm. The substrate may beglass, quartz, alumina, silicon, sapphire, or other refractory materialsthat can sustain the high temperature treatment. Preferably thesubstrate is silicon.

[0038] First “dry” stage:

[0039] With regard to the first “dry” stage, the silica film coated on asubstrate is preferably loaded into a diffusion furnace idling at hightemperature, preferably between 6000 and 1000° C., more preferably at700° C. The silica film coated on a substrate is preferably loadedwithin 5 to 120 minutes, more preferably within 15 minutes. The silicafilm coated on a substrate is preferably loaded under a nitrogen orargon flow, more preferably nitrogen, at a flow rate of 1 to 100 litersof argon or nitrogen per minute, more preferably at a flow rate of about10 liters/minute.

[0040] The temperature to which the silica film is subject is increasedto a maximum temperature. Preferably this temperature increase isperformed a rate of 2.5 to 20° C./minute, more preferably at about 5°C./minute.

[0041] The maximum temperature to which the silica film is subjectedaccording the method of the invention is preferably 600° C. to 1350° C.,more preferably 800° C. to 1200° C. The silica film is preferablyallowed to stabilise at the maximum temperature under the argon ornitrogen atmosphere or under vacuum for 10 to 120 minutes, morepreferably about 30 minutes.

[0042] “Wet” stage:

[0043] With regard to the “wet” stage, the reactive atmospherepreferably comprises a mixture of between 1.0 and 20.0 liter/minute ofoxygen and between 1.0 and 30.0 liter/minute of hydrogen is preferred,more preferably the mixture is about 4.4 liter/minute of oxygen andabout 7.5 liter/minute of hydrogen. Alternatively, the reactiveatmosphere may comprise gases, other than hydrogen (H₂) and oxygen (O₂),which contain hydrogen and oxygen atoms. Examples include water vapour(H₂O), hydrogen peroxide (H₂O₂), and nitric acid (HNO₃). These othergases may be used at a gas flow rate of between 1.0 and 20.0liter/minute.

[0044] The silica film is preferably allowed to be exposed to thesereactive conditions from 30 minutes to 600 minutes, more preferablyabout 120 minutes.

[0045] Second “dry” stage:

[0046] With regard to the second “dry” stage, the reactive gas may bereplaced by a nitrogen or argon flow, more preferably nitrogen, at aflow rate of 1 to 100 liters of argon or nitrogen per minute, morepreferably at a flow rate of about 10 liters/minute.

[0047] The silica film is preferably allowed to stabilise at the maximumtemperature under the argon or nitrogen atmosphere or under vacuum for10 to 120 minutes, more preferably about 30 minutes.

[0048] The temperature to which the silica film is subject is decreasedto a temperature of about 1000° to 600° C. Preferably this temperaturedecrease is performed a rate of 1 to 10° C./minute, more preferably atabout 2.5° C./minute.

[0049] The silica film coated on a substrate is preferably recoveredfrom a diffusion furnace at a temperature of between 600 and 1000° C.,more preferably at 700° C. The silica film coated on a substrate ispreferably recovered from a diffusion furnace within 5 to 120 minutes,more preferably within 15 minutes. The silica film coated on a substrateis preferably recovered from under a flow of nitrogen or argon,preferably nitrogen, at a flow rate of 1 to 100 liters of argon ornitrogen per minute, more preferably at a flow rate of about 10liters/minute.

[0050] For the purpose of manufacturing optical devices which employsilica waveguides, such as optical Multiplexers (Mux) and Demultiplexers(Dmux), the method of this invention is preferably used to reduceabsorbance in the 1.30 and 1.55 μm optical regions. However, the methodof this invention is not limited to the 1.30 and 1.55 μm optical regionssince the higher oscillation harmonics of the eliminated oscillatorsusing the chemical treatment of this invention have other opticalbenefits at longer or shorter wavelengths. This invention alsopreferably encompasses the first, second, third, and fourth harmonics ofthese oscillators.

[0051] The silica films coated on the substrate used in this inventionmay be prepared in a manner known in the art. The silica films areusually deposited by plasma enhanced chemical vapor deposition (PECVD).However, they may also be deposited by flame hydrolysis (FH), lowpressure chemical vapor deposition (LPCVD), metal organic vapordeposition (MOCVD), electron cyclotron resonance deposition (ECRD), orby RF sputtering (RFS). Doped with Phosphorus, Boron, Germanium orTitanium.

[0052] The PECVD silica films could be deposited at a range oftemperatures known to one skilled in the art. Preferably the silicafilms are deposited at about 400° C.

[0053] The silica films are preferably un-doped, but may be doped withphosphorus, boron, germanium, or titanium.

[0054] The silica films of this invention may be used for a number ofpurposes including Mux/Dmux devices, other photonics devices,semiconductor devices, micro-electromechanical systems (MEMS),bio-chips; lab-on-a-chip devices, and multi-chip modules.

COMPARATIVE EXAMPLE

[0055] Un-doped PECVD silica films were subject to a maximum temperatureof between 800° C. and 1200° C. in a BTI 150 mm horizontal diffusionfurnace using a Dry-Dry-Dry three step chemical treatment process. Thethree steps are as follows.

[0056] Dry:

[0057] The silicon wafers, coated with the required combinations ofPECVD silica films, were loaded in a diffusion furnace idling at 700° C.within 15 minutes under 10 liter/minute of nitrogen gas flow. This wasfollowed by a ramp-up of the furnace temperature at a rate of 5°C./minute from 700° C. to the maximum thermal treatment temperatureunder the same 10 liter/minute of nitrogen gas flow. The silicon waferswere allowed to stabilise for 30 minutes at the maximum thermaltreatment temperature under 10 liter/minute of nitrogen gas flow.

[0058] Dry:

[0059] While at the maximum thermal treatment temperature, the 10liter/minute of nitrogen was maintained and the silicon wafers wereallowed to stabilise for 120 minutes at this maximum thermal treatmenttemperature.

[0060] Dry:

[0061] The 10 liter/minute of nitrogen gas flow was maintained and thesilicon wafers were allowed to stabilise for 30 minutes at the maximumthermal treatment temperature. This was followed by a ramp-down of thefurnace temperature at a rate of 2.5° C./minute from the maximum thermaltreatment temperature to 700° C. under the 10 liter/minute of nitrogengas flow. The thermally treated PECVD silica films were then unloadedunder the 10 liter/minute of nitrogen gas flow within 15 minutes.

[0062]FIG. 5 shows the basic FTIR spectra of PECVD silica films beforeand after a three hour long high temperature thermal treatment in anitrogen ambient at a maximum temperature of 600°, 700°, 800°, 900°,1000°, or 1100° C. It is clear that the higher the thermal decompositiontemperature of the high temperature thermal treatment in a nitrogenambient, the better the thermal decomposition of silica films, thebetter the elimination of nitrogen, hydrogen, and ammonia (i.e. as perthe chemical reactions of FIG. 1) and the better the FTIR spectra of thetreated silica films (i.e. the better the four basic optical absorptionpeaks). The spectra of FIG. 5 include several peaks: a more intense andsmaller FWHM Si—O—Si “rocking mode” absorption peak ranging between 410and 510 cm⁻¹; a smaller FWHM Si—O—Si “bending mode” absorption peakranging between 740 and 880 cm⁻¹; a more intense and smaller FWHMSi—O—Si “in-phase-stretching mode” absorption peak ranging between 1000and 1160 cm⁻¹, indicating a more stoichiometric silica films with theoptimum density and optimum Si—O—Si bond angle of 144°; a gradualelimination of the Si—O—Si “out-of-phase-stretching mode” absorptionpeak ranging between 1080 and 1280 cm⁻¹, as compared to the Si—O—Siin-phase-stretching mode absorption peak; a gradual separation betweenthe Si—O—Si “in-phase-stretching mode” absorption peak (1080 cm⁻¹) andthe Si—O—Si “bending mode” absorption peak (810 cm⁻¹) with a deepervalley between 850 and 1000 cm⁻¹.

[0063] From the FTIR spectra from 810 to 1000 cm⁻¹ of FIG. 6 it is clearthat the higher the thermal decomposition temperature, the better thenet separation between the Si—O—Si “in-phase-stretching mode” absorptionpeak (1080 cm⁻¹) and the Si—O—Si “bending mode” absorption peak (810cm⁻¹) and the deeper the valley between 850 and 1000 cm⁻¹. The reductionand gradual elimination of the Si—OH oscillators, centered at 885 cm⁻¹(i.e. of some configurations of the SiOH₂ residual potential compounds)using various chemical reactions of FIG. 2 was demonstrated to occurfollowing the 600° C. thermal treatment in a nitrogen ambient. Aresidual peak was observed at 950 cm⁻¹, indicating the presence ofresidual oscillators as a result of the various thermal decompositionreactions of FIG. 2. These residual oscillators are associated to theSi—ON oscillators of two (2) of the six (6) residual potentialcompounds: SIONH and SiON₂. The higher the temperature, the morenitrogen incorporation and the more evident the Si—ON oscillators (i.e.some configurations of the residual potential; SiONH and/or SiON₂compounds).

[0064] The region from 1500 to 1600 cm⁻¹ of FIG. 7 focuses on the N═Noscillators, centered at 1555 cm⁻¹, of the various compounds describedby the various chemical reactions of FIG. 2. It is clear that the higherthe temperature, the better the elimination of N═N oscillators (whichfifth harmonics could cause an optical absorption between 1.266 and1.307 μm) with a complete elimination of residual N═N oscillators (i.e.some configurations of the residual potential SiON₂ compounds) after athermal treatment beyond 900° C. in a nitrogen ambient.

[0065] The region from 1700 to 2200 cm⁻¹ of FIG. 8 focuses on the Si═Ooscillators, centered at 1875 cm⁻¹ of four (4) of the six (6) residualpotential compounds: SiO₂, SiOH₂, SiONH and SiON₂. Another unknownabsorption peak was also observed centered at 2010 cm⁻¹ but since thisunknown oscillator does not have a higher harmonics which could causeoptical absorption in the 1.30 and 1.55 μm optical bands, the search ofits identity was not prioritized. It is clear that the higher thethermal decomposition temperature, the more evident the Si═O oscillators(which fourth harmonics could cause an optical absorption between 1.282and 1.389 μm) and the more evident the unknown oscillators which have nohigher absorption harmonics between 1.300 and 1.550 μm.

[0066]FIG. 9 shows the in-depth FTIR spectra from 2200 to 2400 cm⁻¹.This region of interest focuses on the Si—H oscillators, centered at2260 cm⁻¹ of three of the six residual potential compounds: SiNH, SiOH₂,and SiONH. It was noted that the higher the thermal decompositiontemperature, the better the elimination of Si—H oscillators (which thirdharmonics could cause an optical absorption between 1.443 and 1.508 μm)with a complete elimination of residual Si—H oscillators (i.e. someconfigurations of the residual potential SiNH, SiOH₂, and SiONHcompounds) after a thermal treatment beyond 600° C. in a nitrogenambient.

[0067]FIG. 10 shows the in-depth FTIR spectra from 3200 to 3900 cm⁻¹.This region of interest focuses on the Si:N—H oscillators, centered at3380 cm⁻¹, the SiN—H oscillators, centered at 3420 cm⁻¹, the SiO—Hoscillators, centered at 3510 cm⁻¹, and the HO—H oscillators, centeredat 3650 cm⁻¹ of three of the six residual potential compounds: SiNH,SiOH₂ and SiONH. It is clear that the higher the thermal decompositiontemperature, the better the elimination of: the HO—H oscillators(trapped water vapour in the micropores of the silica films and whichsecond harmonics could cause an optical absorption between 1.333 and1.408 μm) with a complete elimination over 600° C.; the SiO—Hoscillators (which second harmonics could cause an optical absorptionbetween 1.408 and 1.441 μm) with a complete elimination over 900° C.;the SiN—H oscillators (which second harmonics could cause an opticalabsorption between 1.445 and 1.479 μm)) with a complete elimination over1000° C.; and the Si:N—H oscillators (which second harmonics could causean optical absorption between 1.445 and 1.515 μm) with are not yetcompletely eliminated at 1100° C. The complete elimination of the SiN—Hoscillators is extremely difficult because the nitrogen atom is bondedto the silicon atom of the SiO₂ network with two covalent bonds.

[0068] This comparative experiment demonstrates that it is verydifficult to use the “dry-dry-dry” method in order to completelyeliminate the residual oscillators of various undesirableSi—O_(x)—H_(y)—N_(z) potential compounds and achieve optical qualitysilica films from PECVD silica films using a simple thermaldecomposition thermal treatment at temperature between 600 and 1100° C.

EXAMPLE 1

[0069] UN-doped PECVD silica films were subject to a maximum temperatureof between 800° C. and 1200° C. in a BTI 150 mm horizontal diffusionfurnace using a Dry-Wet-Dry three step chemical treatment process:

[0070] Dry:

[0071] The silicon wafers, coated with the required combinations ofPECVD silica films, were loaded into a diffusion furnace idling at 700°C. within 15 minutes under 10 liter/minute of nitrogen gas flow,followed by a ramp-up of the furnace temperature at a rate of 5°C./minute from 700° C. to the maximum thermal treatment temperatureunder the same 10 liter/minute of nitrogen gas flow and by astabilisation for 30 minutes at the maximum thermal treatmenttemperature under the 10 liter/minute of nitrogen gas flow;

[0072] Wet:

[0073] While at the maximum thermal treatment temperature, the 10liter/minute of nitrogen gas flow was replaced by a reactive gas flowcomposed of 4.38 liter/minute of oxygen and 7.5 liter/minute of hydrogenand stabilised for 120 minutes in this gas mixture at a maximum thermaltreatment temperature;

[0074] Dry:

[0075] The oxygen/hydrogen gas mixture was replaced by a 10 liter/minuteof nitrogen gas flow, followed by a stabilisation at the maximum thermaltreatment temperature for 30 minutes under the 10 liter/minute ofnitrogen gas flow, followed by a ramp-down of the furnace temperature ata rate of 2.5° C./minute from the maximum thermal treatment temperatureto 700° C. under the 10 liter/minute of nitrogen gas flow, followed bythe unloading of the thermally treated combination of PECVD silica filmsunder the 10 liter/minute of nitrogen gas flow within 15 minutes.

[0076]FIG. 12 shows the basic FTIR spectra of PECVD silica films beforeand after a three hour long high temperature chemical treatment in a H₂,O₂, and H₂O ambient at a temperature of either 800°, 900°, 1000°, or1100° C. as well as after a three hour long high temperature thermaltreatment in a N₂ ambient at a temperature of 1100° C. from thecomparative example described above. It is clear that the higher thetemperature, the better the elimination of: nitrogen, hydrogen, andammonia (i.e. as per the chemical reactions of FIG. 4), and the betterthe FTIR spectra of the treated silica films (i.e. the better the fourbasic optical absorption peaks). Thus, FIG. 12 shows a much more intenseand smaller FWHM Si—O—Si “rocking mode” absorption peak (between 410 and510 cm⁻¹) with the lowest temperature chemical treatment of 800° C.showing a more intense and smaller FWHM Si—O—Si “rocking mode”absorption peak then the best result achieved with the thermal treatmentof the comparative example at 1100° C.; a more intense and smaller FWHMSi—O—Si “bending mode” absorption peak (between 740 and 880 cm⁻¹) withthe lowest temperature chemical treatment of 800° C. showing a moreintense and smaller FWHM Si—O—Si “bending mode” absorption peak than thebest result achieved with the thermal treatment at 1100° C.; a much moreintense and smaller FWHM Si—O—Si “in-phase-stretching mode” absorptionpeak (between 1000 and 1160 cm⁻¹) indicating a more stoichiometricsilica film with the optimum density and optimum Si—O—Si bond angle of1440 with the lowest temperature chemical treatment of 800° C. showing amore intense and smaller FWHM Si—O—Si “in-phase-stretching mode”absorption peak then the best result achieved with the thermal treatmentat 1100° C.; a gradual and impressive elimination of the Si—O—Si“out-of-phase-stretching mode” absorption peak (between 1080 and 1280cm⁻¹), as compared to the Si—O—Si in-phase-stretching mode absorptionpeak with the lowest temperature chemical treatment of 800° C. showing amore complete elimination of the Si—O—Si “out-of-phase-stretching mode”absorption peak than the best result achieved with the thermal treatmentat 1100° C.; a very clean separation between the Si—O—Si“in-phase-stretching mode” absorption peak (1080 cm⁻¹) and the Si—O—Si“bending mode” absorption peak (810 cm⁻¹) with a very deep valleybetween 850 and 1000 cm⁻¹ with the lowest temperature chemical treatmentof 800° C. showing a better separation between the Si—O—Si“in-phase-stretching mode” absorption peak and the Si—O—Si “bendingmode” absorption peak and a deeper valley between 850 and 1000 cm⁻¹ thanthe best result achieved with the thermal treatment at 1100° C.

[0077] A close-up of some infrared regions of the FTIR spectra of FIG.11 with the help of the FTIR regions of the table of FIG. 3 verifies theelimination of the various Si—O_(x)—H_(y)—N_(z) potential compounds andverifies the achievement of pure SiO₂ with minimum optical absorption inthe 1.30 and 1.55 μm optical bands.

[0078]FIG. 12 shows a close-up of the FTIR spectra from 810 to 1000cm⁻¹. It is clear that the higher the temperature, the better the netseparation between the Si—O—Si “in-phase-stretching mode” absorptionpeak (1080 cm⁻¹) and the Si—O—Si “bending mode” absorption peak (810cm⁻¹) and the deeper the valley between 850 and 1000 cm⁻¹. The reductionand total elimination of the Si—OH oscillators (centered at 885 cm⁻¹)using various chemical reactions of FIG. 4 is more complete as thetemperature of the chemical treatment is increased. In fact, it isdemonstrated that the elimination of the Si—OH oscillators is even morecomplete with the lowest temperature chemical treatment of 800° C. thanwith the best thermal treatment at 1100° C. Similarly, the gradualreduction and elimination of the Si—ON oscillators (centered at 950cm⁻¹) using various chemical reactions of FIG. 4 is more complete as thetemperature of the chemical treatment is increased. Unlike the thermaltreatment in nitrogen of the comparative example which tends toincorporate more nitrogen and form more Si—ON oscillators (i.e. moreresidual potential SiONH and/or SiON₂ compounds) as the temperature ofthe thermal treatment is increased, the chemical treatment of theinvention eliminates more and more of the Si—ON oscillators as thetemperature of the chemical treatment is increased. Again, it isdemonstrated that the elimination of the Si—ON oscillators is even morecomplete with the lowest temperature chemical treatment of 800° C. thanwith the best thermal treatment at 1100° C. The net separation and deepvalley indicate that the silica films resulting from these hightemperature chemical treatments are composed of high quality SiO₂material.

[0079]FIG. 13 shows a close-up of the FTIR spectra from 1500 to 1600cm⁻¹. This region of interest focuses on the N═N oscillators (centeredat 1555 cm⁻¹ and which fifth harmonics could cause an optical absorptionbetween 1.266 and 1.307 μm) of the various compounds described by thevarious chemical reactions of FIG. 4. It is clear that these oscillatorsare completely eliminated by the high temperature chemical treatmentsand that a chemical treatment at a temperature of 800° C. is asefficient as the best thermal treatment at 1100° C. for the eliminationof residual N═N oscillators.

[0080]FIG. 14 shows a close-up of the FTIR spectra from 1700 to 2200cm⁻¹. This region of interest focuses on the Si═O oscillators (centeredat 1875 cm⁻¹) and on the unknown oscillator (centered at 2010 cm⁻¹) ofthe various compounds described by the various chemical reactions ofFIG. 4. It is clear that the higher the temperature from 800 to 1100° C.in a H₂, O₂, and H₂O ambient, the more evident the Si═O oscillators(which fourth harmonics could cause an optical absorption between 1.282and 1.389 μm) and the unknown oscillators (which does not have a higherharmonics which could cause optical absorption in the 1.30 and 1.55 μmoptical bands).

[0081]FIG. 15 shows a close-up of the FTIR spectra from 2200 to 2400cm⁻¹. This region of interest focuses on the Si—H oscillators (centeredat 2260 cm⁻¹) of the various compounds described by the various chemicalreactions of FIG. 4. It is clear that these oscillators are completelyeliminated by the high temperature chemical treatments and that achemical treatment at a temperature of 800° C. is as efficient as thebest thermal treatment at 1100° C. for the elimination of residual Si—Hoscillators (which third harmonics could cause an optical absorptionbetween 1.443 and 1.508 μm).

[0082]FIG. 16 shows a close-up of the 3200 to 3900 cm⁻¹ region of theFTIR spectra of FIG. 11. This region of interest focuses on the Si:N—Hoscillators (centered at 3380 cm⁻¹), on the SiN—H oscillators (centeredat 3420 cm⁻¹), on the SiO—H oscillators (centered at 3510 cm⁻¹) and onthe HO—H oscillators (centered at 3650 cm⁻¹). It is clear that all theseoscillators are completely eliminated by the high temperature chemicaltreatments and that a chemical treatment at a temperature of 800° C. isas efficient as the best thermal treatment at 1100° C. for theelimination of: the HO—H oscillators (trapped water vapour in themicropores of the silica films and which second harmonics could cause anoptical absorption between 1.333 and 1.408 μm); the SiO—H oscillators(which second harmonics could cause an optical absorption between 1.408and 1.441 μm); the SiN—H oscillators (which second harmonics could causean optical absorption between 1.445 and 1.479 μm); the Si:N—Hoscillators (which second harmonics could cause an optical absorptionbetween 1.445 and 1.515 μm) are completely eliminated with a hightemperature chemical treatment at a temperature as low as 800° C. Thiscontrasts with their incomplete elimination using thermal decompositionin a thermal treatment at 1100° C. due to the difficulty of thermallybreaking the two covalent bonds binding the nitrogen atom to the siliconatom of the SiO₂ network.

[0083] It should be obvious that the above described embodiments aremerely illustrative of the application and of the principles of thepresent invention, and numerous modification thereof may be devised bythose skilled in the art without departing from the spirit and scope ofthe invention.

REFERENCES

[0084] The following references are incorporated herein.

[0085]¹ Hoffmann M., Low temperature, nitrogen doped waveguides onsilicon with small core dimensions fabricated by PECVD/RIE, ECIO'95,299, 1995; Bazylenko M., Pure and fluorine-doped silica films depositedin a hollow cathode reactor for integrated optic applications, J. Vac.Sci. Technol. A 14 (2), 336, 1996; Uchida N., Passively aligned hybridWDM module integrated with spot-size converter integrated laser diodeand waveguide photodiode on PLC platform for fibre-to-the-home,Electronic Letters, 32 (18), 1996; Inoue Y., Silica-based planarlightwave circuits for WDM systems, IEICE Trans. Electron., E80C (5),1997; Inoue Y., PLC hybrid integrated WDM transceiver module for accessnetworks, NTT Review, 9 (6), 1997; Hoffmann M., Low-loss fiber-matchedlow-temperature PECVD waveguides with small-core dimensions for opticalcommunication systems, IEEE Photonics Tech. Lett., 9 (9), 1238, 1997;Takahashi H., Arrayed-waveguide grating wavelength multiplexers for WDMsystems, NTT Review, 10 (1), 1998; Himeno A., Silica-based planarlightwave circuits, IEEE J. of Selected Topics in Quantum Electronics, 4(6), 1998.

[0086]² Imoto K., Silica Glass waveguide structure and its implicationto a multi/demultiplexer, ECOC, 577, 1988; Verbeek B., Integratedfour-channel Mach-Zehnder multi-demultiplexer fabricated with phosphorusdoped SiO2 waveguides on Si, J. Lightwave tech., 6 (6), 1011, 1988;Henry C., Glass waveguides on silicon for hybrid optical packaging, J.Lightwave tech., 7 (10), 1350, 1989; Grand G., Low-loss PECVD silicachannel waveguides for optical communications, Electron. Lett., 26 (25),2135, 1990; Bruno F., Plasma-enhanced chemical vapor deposition oflow-loss SiON optical waveguides at 1.5-μm wavelength, Applied Optics,30 (31), 4560, 1991; Kapser K., Rapid deposition of high-qualitysilicon-oxinitride waveguides, IEEE Trans. Photonics Tech. Lett., 5(12), 1991; Lai Q., Simple technologies for fabrication of low-losssilica waveguides, Elec. Lett., 28 (11), 1000, 1992; Schneider H.,Realization of SiO2-B2O3-TiO2 waveguides and reflectors on Sisubstrates, Mat. Res. Soc. Symp. Proc. Vol 244, 377, 1992; Jai Q.,Formation of optical slab waveguides using thermal oxidation of SiOx,Electronic Letters, 29 (8), 1993; Imoto K., High refractive indexdifference and low loss optical waveguide fabricated by low temperatureprocesses, Electronic Letters, 29 (12), 1993; Tu Y., Single-modeSiON/SiO2/Si optical waveguides prepared by plasma-enhanced Chemicalvapor deposition, Fiber and integrated optics, 14, 133, 1995; BazylenkoM., Pure and fluorine-doped silica films deposited in a hollow cathodereactor for integrated optic applications, J. Vac. Sci. Technol. A 14(2), 336, 1996; Kawachi M., Recent progress in silica-based planarlightwave circuits on silicon, IEE Proc. Optoelectron., 143 (5), 1996;Hoffmann M., Low-loss fiber-matched low-temperature PECVD waveguideswith small-core dimensions for optical communication systems, IEEEPhotonics Tech. Lett., 9 (9), 1238, 1997; Alayo M., Thick SiOxNy andSiO2 films obtained by PECVD technique at low temperatures, Thin SolidFilms, 332, 40, 1998; Johnson C., Thermal annealing of waveguides formedby ion implantation of silica-on-Si, Nuclear Instruments and Methods inPhysics Research, B141, 670, 1998; Himeno A., Silica-based planarlightwave circuits, IEEE J. of Selected Topics in Quantum Electronics, 4(6), 1998; Ridder R., Silicon oxynitride planar waveguiding structuresfor application in optical communication, IEEE J. of Sel. Top. InQuantum Electron., 4 (6), 930, 1998 Germann R., Silicon-oxynitridelayers for optical waveguide applications, 195^(th) meeting of theElectrochemical Society, 99-1, May 1999, Abstract 137, 1999; Worhoff K.,Plasma enhanced chemical vapor deposition silicon oxynitride optimizedfor application in integrated optics, Sensors and Actuators, 74, 9,1999.

[0087]³ Grand G., Low-loss PECVD silica channel waveguides for opticalcommunications, Electron. Lett., 26 (25), 2135, 1990; Bruno F.,Plasma-enhanced chemical vapor deposition of low-loss SiON opticalwaveguides at 1.5-μm wavelength, Applied Optics, 30 (31), 4560, 1991;Imoto K., High refractive index difference and low loss opticalwaveguide fabricated by low temperature processes, Electronic Letters,29 (12), 1993; Hoffmann M., Low temperature, nitrogen doped waveguideson silicon with small core dimensions fabricated by PECVD/RIE, ECIO'95,299, 1995; Bazylenko M., Pure and fluorine-doped silica films depositedin a hollow cathode reactor for integrated optic applications, J. Vac.Sci. Technol. A 14 (2), 336, 1996; Pereyra I., High quality lowtemperature DPECVD silicon dioxide, J. Non-Crystalline Solids, 212, 225,1997; Kenyon A., A luminescence study of silicon-rich silica andrareearth doped silicon-rich silica, Electrochem. Soc. Proc. Vol. 97-11,304, 1997; Alayo M., Thick SiOxNy and SiO2 films obtained by PECVDtechnique at low temperatures, Thin Solid Films, 332, 40, 1998; WorhoffK., Plasma enhanced chemical vapor deposition silicon oxynitrideoptimized for application in integrated optics, Sensors and Actuators,74, 9, 1999; Germann R., Silicon-oxynitride layers for optical waveguideapplications, 195^(th) meeting of the Electrochemical Society, 99-1, May1999, Abstract 137, 1999.

1. A method for reducing the optical absorbance of a silica film coated on a substrate, comprising: (a) subjecting the silica film coated on the substrate to a temperature of about 600° to about 1000° C. under nitrogen, under an inert atmosphere, or under vacuum; (b) increasing the temperature to a maximum temperature of at most 1350° C.; (c) exposing the silica film to a reactive atmosphere comprising oxygen atoms and hydrogen atoms by replacing the nitrogen, inert atmosphere, or vacuum by the reactive atmosphere; (d) removing the silica film from the reactive atmosphere by replacing the reactive atmosphere with a nitrogen, an inert atmosphere, or vacuum; (e) decreasing the temperature from the maximum temperature to about 600° to about 1000° C.; and (f) recovering the silica film coated on the substrate.
 2. The method according to claim 1, wherein the silica film is coated onto the substrate by a method selected from the group consisting of: flame hydrolysis, plasma enhanced chemical vapour deposition, low pressure chemical vapour deposition, metal organic vapour deposition, electron cyclotron resonance deposition, and RF Sputtering.
 3. The method according to claim 2, wherein the silica film is coated on the substrate by plasma enhanced chemical vapour deposition.
 4. The method according to claim 1, wherein the silica film is doped with an element selected from the group consisting of phosphorus, boron, germanium, and titanium.
 5. The method according to claim 1, wherein the optical absorbance is reduced in the 1.30 and 1.55 μm optical bands.
 6. The method according to claim 1, wherein the maximum temperature is 800° to 1200° C.
 7. The method according to claim 1, wherein the method is carried out in a diffusion furnace.
 8. The method according to claim 7, wherein the diffusion furnace is a BTI horizontal furnace.
 9. The method according to claim 1, wherein the substrate is a 100 mm, 125 mm, 150 mm, 200 mm, or 300 mm wafer.
 10. The method according to claim 9, wherein the wafer is a 150 mm wafer.
 11. The method according to claim 1, wherein the substrate is silicon.
 12. The method according to claim 1, wherein the silica film coated on the substrate is placed in a diffusion oven at a temperature of about 700° C.
 13. The method according to claim 1, wherein the silica film coated on the substrate is loaded into a diffusion oven within about 5 to 120 minutes.
 14. The method according to claim 1, wherein in step (a), the silica film coated on the substrate is loaded into a diffusion oven under nitrogen flow.
 15. The method according to claim 1, wherein in step (b), the temperature is increased at a rate of about 2.50 to about 20° C./minute.
 16. The method according to claim 1, wherein after step (b), the silica film is allowed to stabilise for about 10 to about 120 minutes.
 17. The method according to claim 1, wherein the reactive atmosphere comprises flows of O₂ and H₂.
 18. The method according to claim 1, wherein the reactive atmosphere comprises a flow of a gas selected from water vapour, hydrogen peroxide, and nitric acid.
 19. The method according to claim 1, wherein in step (d), the reactive atmosphere is replaced with a nitrogen flow.
 20. The method according to claim 1, wherein in step (d), the silica film is allowed to stabilise under the nitrogen atmosphere, argon atmosphere, or under a vacuum for a period of 10 to 120 minutes.
 21. The method according to claim 1, wherein in step (e), the temperature is decreased at a rate of about 10 to about 10° C./minute.
 22. The method according to claim 1, wherein in step (f), the silica film is recovered from a diffusion furnace within 5 to 120 minutes.
 23. The method according to claim 1, wherein in step (f), the silica film is recovered under a nitrogen flow.
 24. A method of making a high optical quality silica film, comprising: a) subjecting a substrate to a gaseous mixture of silane and nitrous oxide, to deposit said film on said substrate in accordance with the reaction SiH₄(g)+2N₂O(g)→SiO₂+2N₂(g)+2H₂(g) and b) subsequently subjecting said deposited film to a reactive atmosphere containing hydrogen and oxygen atoms to chemically transform impurities resulting from the reaction into pure silica. 